Memories such as an SRAM and a DRAM have already been used in a variety of electronic devices. For example, an SRAM or a DRAM is widely used as a cache memory. A cache memory is used in a central processing unit (CPU), a hard disk drive (HDD), a solid state drive (SSD), or the like. A cache memory is provided to input or output data to a logic circuit quickly.
Although an example of nonvolatile memories includes a flash memory, response speed of a flash memory is low. As a novel memory, a memory using an oxide semiconductor has been proposed.
The off-state current of a transistor using an oxide semiconductor is extremely low. For example, the off-state current per micrometer of the channel width at 85° C. is less than or equal to 100 yA (1 yA (yoctoampere) is 1×10−24 A). Thus, a memory element which can store data for a long period can be formed using a storage capacitor and an oxide semiconductor transistor (e.g., Patent Document 1).
A technique of controlling a threshold voltage of an oxide semiconductor transistor is still under development. In the case where the threshold voltage is shifted in the negative direction, the off-state current is increased at Vg=0 V. Owing to the shift in the threshold voltage, a data retention period is shortened. Therefore, in view of this point, the technique of controlling a threshold voltage is required for lengthening a data retention period.